As the density of semiconductor devices increases and the size of circuit elements becomes smaller, the resistance capacitance (RC) delay time increasingly dominates the circuit performance. To reduce the RC delay, the demands on interconnect layers for connecting the semiconductor devices to each other increases. Therefore, there is a desire to switch from traditional silicon dioxide based dielectrics to low-k dielectrics. These materials are particularly useful as intermetal dielectrics, IMDs, and as interlayer dielectrics, ILDs.
One example of a low-k material is fluorine-doped silicon dioxide, or fluorosilicate glass (FSG). Another widely used material is a carbon-doped oxide or organosilicate glass (OSG). OSG films typically comprise SiwCxOyHz wherein the tetravalent silicon may have a variety of organic group substitutions. A commonly used substitution creates methyl silsesquioxane (MSQ), wherein a methyl group creates a SiCH3 bond in place of a SiO bond. There are several approaches known in the art for reducing the k-value of dielectric films. These include decreasing the film density, reducing the film ionization, and reducing the film polarization.
Since air has a dielectric constant of about 1, one method for making low-k dielectrics incorporates air into dense materials to make them porous. The dielectric constant of the resulting porous material is combination of the dielectric constant of air and the dielectric constant of the dense material. Therefore, it is possible to lower the dielectric constant of current low-k materials by making them porous. Silica based xerogels and aerogels, for example, incorporate a large amount of air in pores or voids, thereby achieving dielectric constants less than 1.95 with pores are as small as 5-10 nm.
A major drawback with porous dielectrics, however, is that they are susceptible to damage from plasma etching and ashing processes used in device fabrication. Porous dielectrics are also softer than conventional dielectrics so they are more easily damaged during aggressive handling operations like chemical mechanical polishing. Damaged dielectrics often have surface fractures that allow processing chemicals or moisture to enter the internal porous network, thereby causing corrosion, mechanical damage, or an increase in the dielectric constant. Such damage may decrease reliability without any direct methods of detection. Pore damage may also cause a surface that is preferably hydrophobic to become hydrophilic, which may alter the wetability of various solvents.
In light of problems such as these, there remains a need for low-k dielectrics that not only have a porous structure, but that also have the chemical and mechanical stability to withstand the harsh manufacturing steps.